In a conventional image forming apparatus of electrophotographic type, a uniform electric charge of about 1 millicoulomb per square centimeter is applied on the surface of a photosensitive drum. This photosensitive drum is exposed depending on the image information, and the electric charge of the irradiated portion is released to the photosensitive drum substrate, and an image (electrostatic latent image) is formed as a result of distribution of electric charge. This electrostatic latent image is developed by coloring charged particles (toner particles), and a powder image is formed (toner development). The powder image is transferred onto a sheet material or the like. The transferred powder image is fused and fixed by heat or other energy, and formed as an image.
Recently, on the other hand, the image by the image forming apparatus of electrophotographic type is becoming colorful. As the color image forming apparatus, a color image forming apparatus of tandem engine type is proposed. This color image forming apparatus includes a plurality of image forming stations having photosensitive drums. The plurality of image forming stations are image forming stations for forming cyan image, magenta image, yellow image, and, preferably, black image on each photosensitive drum. The powder image formed on each photosensitive drum is overlaid on an intermediate transfer material at transfer position of each color powder image, and transferred and synthesized. In the color image forming apparatus of tandem engine type, the image can be formed at high speed because each image is formed parallel in each color.
In the case of such color image forming apparatus, each powder image formed in a different image forming station may be deviated in position due to positioning error between image forming stations, resulting in color deviation. To develop a color image forming apparatus of high quality, such color deviation is a serious problem, and the technique for correcting color deviation (registration) is required.
In the first place, a reference pattern for detecting color deviation (hereinafter called registration pattern) is drawn. The registration pattern is detected by a plurality of sensors (color deviation detection), and the deviation amount is detected from this result. On the basis of the calculated deviation amount, each image is positioned (color deviation correction).
The operation and color deviation detection of the conventional color image forming apparatus are described below.
FIG. 10 is a structural diagram showing a general color image forming apparatus, FIG. 11 is a structural diagram showing a color deviation detector, FIG. 12 is a layout diagram showing configuration of registration pattern and color deviation detector on an intermediate transfer material, and FIG. 13 is a block diagram showing a conventional drive unit.
First, the configuration in FIG. 10 is explained. The image forming apparatus has four image stations 21k, 21y, 21m, 21c. The image stations 21k, 21y, 21m, 21c have photosensitive drums 22k, 22y, 22m, 22c, respectively.
Around the circumference of the photosensitive drums 22k, 22y, 22m, 22c, the following components are disposed:
a) chargers 23k, 23y, 23m, 23c, PA1 b) exposure units 26k, 26y, 26m, 26c of scanning optical system for irradiating the photosensitive drums 22k, 22y, 22m, 22c with laser beams 29k, 29y, 29m, 29c depending on the image information, PA1 c) developing units 24k, 24y, 24m, 24c, PA1 d) transfer units 28k, 28y, 28m, 28c in a transfer section 27, and PA1 e) cleaning units 25k, 25y, 25m, 25c. PA1 1) Unstable temperature when turning on the power. PA1 2) Exchange of image forming stations 21k, 21y, 21m, 21c. PA1 3) Setting condition of the image forming apparatus. PA1 4) Deviation of the image forming stations 21k, 21y, 21m, 21c due to temperature changes in the apparatus. PA1 5) Deviation of mounting of scanning optical system. PA1 Variation of rotation of drive motor for driving, PA1 Uneven pitch occurring in the transmission gear train for transmitting the driving force of the drive motor, PA1 Speed fluctuations due to eccentric rotation of the gear or speed fluctuations due to eccentric rotation of the photosensitive drum itself, and others. PA1 Variation of rotation of drive motor for driving, PA1 Uneven pitch occurring in the transmission gear train for transmitting the driving force of the drive motor, PA1 Speed fluctuations due to eccentric rotation of the gear or eccentric rotation of the drive roller, and others. PA1 a) a plurality of image stations including photosensitive drums and developing unit for developing the latent images formed on the photosensitive drums as sensible toner images, PA1 b) a transfer unit for transferring and conveying the toner images formed in the plurality of image stations onto an intermediate transfer material, PA1 c) a plurality of exposure units for irradiating the individual photosensitive drums in the plurality of image stations with light for forming latent images, PA1 d) a plurality of drive motors for driving the plurality of photosensitive drums independently, PA1 e) a plurality of motor rotation controllers for controlling rotation and driving of the plurality of drive motors independently, PA1 f) a plurality of rotation phase detectors for detecting the rotation phase of each one of the plurality of photosensitive drums rotated and driven by the drive motors, PA1 g) a rotation phase difference calculator for calculating the rotation phase difference of the other photosensitive drums corresponding to the rotation phase of the specified photosensitive drum as the reference among the rotation phases detected by the plurality of rotation phase detectors, PA1 h) a phase correction setting unit for setting the rotation phase difference in printing operation, and PA1 i) a phase correction controller for correcting rotation phase of the photosensitive drum on the basis of the calculated rotation phase difference and the set rotation phase difference.
Herein, the image stations 21k, 21y, 21m, and 21c are the units for forming black image, yellow image, magenta image, and cyan image, respectively. On the other hand, so as to pass through the image stations 21k, 21y, 21m, and 21c, an intermediate transfer belt 32 is disposed beneath the photosensitive drums 22k, 22y, 22m, 22c, and moves in the direction of arrow A.
In the conventional color image forming apparatus having such constitution, the image forming operation is as follows.
First, at the image forming station 21k, the surface of the photosensitive drum 22k is uniformly charged with an electrostatic charge by the charger 23k.
Then, an electrostatic latent image corresponding to image information of black component is formed on the photosensitive drum 22k by means of the exposure unit 26k.
This electrostatic latent image is developed on the photosensitive drum 22k as a powder image by black toner particles by the developing unit 24k.
This powder image is transferred on the intermediate transfer belt 32 as a black toner image by the transfer unit 28k.
The surface of the photosensitive drum 22k after transfer process is cleaned by the cleaning unit 25k, and residual toner particles are removed to be ready for next image formation.
On the other hand, parallel to the timing of formation of the black toner image, at the image forming station 21y, the surface of the photosensitive drum 22y is uniformly charged with an electrostatic charge by the charger 23y, and an electrostatic latent image corresponding to the image information of yellow component is formed on the photosensitive drum 22y by the exposure unit 26y.
This electrostatic latent image is developed on the photosensitive drum 22y as a powder image by yellow toner particles by the developing unit 24y, and it is laid over the black toner image formed on the intermediate transfer belt 32, and formed as a synthetic toner image.
Similarly, thereafter, a magenta toner image is overlaid by the image forming station 21m, and a cyan toner image by the image forming station 21c sequentially on the intermediate transfer belt 32. In this way, the synthetic toner image is formed by overlaying four color toner images on the intermediate transfer belt 32.
After the transfer process, the photosensitive drums 22k, 22y, 22m, 22c are cleaned by the cleaning units 25k, 25y, 25m, 25c, and residual toner particles are removed to be ready for next image formation, and the printing operation is finished.
After completion of formation of the synthetic toner image, a sheet material 35 of paper or the like is supplied in between the intermediate transfer belt 32 and a transfer roller 37 from a paper feed cassette 34 through a paper feed roller 36. The transfer roller 37 is disposed at a position contacting with the intermediate transfer belt 32 for inserting the sheet material 35 between it and the intermediate transfer belt 32. When the sheet material 35 is supplied, the synthetic toner image is transferred on the sheet material 35. Then, after being heated and fixed by a fixing unit 38, a color image is formed on the sheet material 35.
As shown in FIG. 13, the drive unit is controlled depending on a control signal from a control unit 50 such as CPU for controlling the operation of the entire apparatus, and a motor rotation controller 51 starts a drive motor 52, and controls its rotating speed. A drive transmission unit 53 transmits the drive force to a rotation moving unit 54 by gear or the like from a rotary shaft of the drive motor 52. By this driving force, the rotation moving unit 54 including the photosensitive drums 22k, 22y, 22m, 22c, intermediate transfer belt 32, heating roller 38a in the fixing unit 38, and others is rotated and driven.
As the drive motor 52, when a known stepping motor (not shown) is used, the motor rotation controller 51 controls the rotating speed by issuing a control signal of frequency corresponding to the rotating speed.
On the other hand, when a DC motor (not shown) is used as the drive motor 52, the motor rotation controller 51 controls the rotating speed of the drive motor by, for example, phase locked loop "PLL" control system. That is, the rotation controller 51 detects an FG signal (a) for generating a frequency proportional to the rotating speed of the drive motor 52, and controls so that the phase and frequency of the FG signal (a) may coincide with the reference clock frequency (not shown), thereby controlling at constant speed rotation.
In such image forming apparatus of tandem engine type, however, color deviation may occur in the following cases:
In the event of color deviation as mentioned above, correction of color deviation in the conventional image forming apparatus is described below.
As shown in FIG. 10, a sensor unit 39 for detecting color deviation is disposed at the downstream side of the image forming stations 21k, 21y, 21m, 21c. As shown in FIG. 11, the sensor unit 39 is composed of a CCD 40, a light source 41 such as lamp, and a SELFOC lens array 42 for focusing the reflected light on the CCD 40.
As shown in FIG. 12, the sensor unit 39 is disposed on the line of the pixels 40a, 40b in the CCD 40 crossing orthogonally with the conveying direction A of the intermediate transfer belt 32. The sensor unit 39 is disposed at two positions, near the image forming start position and image forming end position on the intermediate transfer belt 32.
In such constitution, the detecting operation of color deviation is described below.
Same as in the printing operation, a registration pattern of preliminarily specified line or pattern is formed. For example, as shown in FIG. 12, color toner images 43, 44, 45, 46 are transferred at prescribed intervals between a dotted line 47 including the scanning start position of the exposure unit and a dotted line 48 including the scanning end position on the line crossing orthogonally with the running direction A of the intermediate transfer belt 32. Sensor units 39a and 39b measure the amount of position deviation of each color (color deviation). For example, the position deviation in the main scanning direction (vertical direction to direction A in FIG. 12) is detected as the error from the predetermined design value by detecting the writing start position of main scanning direction of each color when the color registration patterns 43, 44, 45, 46 on the intermediate transfer belt 32 pass through the CCD 40a in the sensor unit 39a. The position deviation in the sub-scanning direction (direction A in FIG. 12) is detected by operating the position deviation of each color (.DELTA.Y1=.DELTA.T1.multidot.v) from the time difference (.DELTA.T=T-T1, where T is the predetermined design value) of time T1 of color registration patterns 43, 44, 45, 46 on the intermediate transfer belt 32 passing through the CCD 40a in the sensor unit 39a and the predetermined design value, and the conveying speed v. In other skew error (inclination in main scanning direction) and multiplication error in main scanning direction (error of print region width in main scanning direction), they can be detected by forming registration patterns in specified shape corresponding to each, and detecting and operating.
In various color deviations thus detected, the correction operation is described below.
For position deviation in the main scanning direction, the control unit 33 for determining the writing start position in the main scanning direction controls the image data writing start timing of the exposure units 26k, 26y, 26m, 26c independently for each color. In this manner, the writing start position in the main scanning direction is corrected.
For position deviation in the sub-scanning direction, the writing timing signal in the sub-scanning direction showing the print region in the sub-scanning direction is controlled independently for each color. Thus, the printing region in the sub-scanning direction is controlled, and the position deviation in the sub-scanning direction is corrected.
Further, skew error and main scanning direction error are corrected by using the image processing technique.
However, the position deviation in the sub-scanning direction is corrected by detecting the position deviation regarded to be always a specific position deviation in the pattern of each color at any position when the registration pattern is formed repeatedly (hereinafter called "position deviation of DC component"). However, the position deviation of the registration pattern due to period fluctuations (hereinafter called "position deviation of AC component") occurring due to the factor explained below cannot be corrected. This position deviation of AC component occurs due to speed fluctuations of the surface of photosensitive drum of each color, speed fluctuations of the surface of intermediate transfer belt, etc.
Speed fluctuations of the surface of photosensitive drum of each color are caused by:
In this case, the position deviation of AC component occurs as fluctuation in the period of the peripheral length of photosensitive drum. The AC component of each color varies in the fluctuation phase.
Speed fluctuations of the surface of intermediate transfer belt are caused by:
In this case, the position deviation of AC component occurs as fluctuation in the period of the peripheral length of intermediate transfer belt.
In the conventional color image forming apparatus, position deviation of DC component was corrected. However, in the event of position deviation of AC component in the sub-scanning direction by speed fluctuation of the photosensitive drum or intermediate transfer belt as mentioned above, it was a problem of the conventional color image forming apparatus that the print quality deteriorates.